1. Field of the Invention
The present invention relates to a liquid phase epitaxial growth method and an apparatus used for carrying out the same.
2. Description of the Related Art
Liquid phase epitaxial growth method (hereinafter referred to as "liquid phase growth method") is a method of growing an epitaxial layer (growth layer) on a single crystal substrate in which the substrate is brought into contact with a saturated solution (or supersaturated solution) of a crystalline component (solute) in a solvent, and subsequently, those are cooled gradually to achieve growth of the epitaxial layer by precipitating the sotute in the solution onto the substrate. For instance, in a liquid phase growth method for GaP, a saturated Ga solution, wherein a GaP crystal (solute) is dissolved in a molton Ga (solvent), is placed by contact on a GaP substrate and then gradually cooled down to a predetermined temperature to achieve growth of a GaP layer by precipitating the GaP in the Ga solution onto the GaP substrate.
In order to obtain a liquid phase growth layer with a uniform layer thickness deposited on a number of substrates, a liquid phase growth apparatus of the closed rotary type (hereinafter referred to as "closed rotary boat") has been used heretofore. FIG. 7 of the accompanying drawings shows, in cross section, one such known closed rotary boat disclosed in Japanese Patent Laid-open Publication No. 57-85222. In FIG. 7, reference character 11 denotes a substrate container in which are disposed a plurality of substrate holders 12.sub.1, 12.sub.2, . . . extending perpendicular to the bottom of the container 11. The substrate holders 12.sub.1, 12.sub.2, . . . each have opposite surfaces to which a pair of substrate crystals 13.sub.1, 13.sub.2, . . . is fixed, respectively. Designated by 14 is a Ga solution container for holding therein a Ga solution 15. Between the substrate container 11 and the Ga solution container 14 there is provided an elongated sliding plate 16 which separate these two containers 11 and 14. The sliding plate 16 is slidable in arrowed direction b shown in FIG. 7, and has a pair of parallel spaced longitudinal grooves 17.sub.1 and 17.sub.2 shown in FIG. 8. Thus, when the sliding plate 16 is slided and a portion with the grooves 17.sub.1, 17.sub.2 is disposed within both containers 11, 14, the boat is half-revolved whereupon the Ga solution 15 is caused to move between both containers 11, 14 through the grooves 17.sub.1, 17.sub.2. Conversely, when a portion without the grooves 17.sub.1, 17.sub.2 of sliding plate 16 is located within both containers 11, 14, move communication between the substrate container 11 and the Ga solution container 14 is blocked by this grooveless portion of the sliding plate 16. Numeral 19 denotes an actuating rod to slide the sliding plate 16, and 18 an engagement hole formed in an end of the sliding plate 16 for receiving therein an end of the actuating rod 19.
Now, description will be given of a process in which the closed rotary boat of the foregoing construction is used to achieve liquid phase growth of a GaP layer.
A plurality of substrate crystals 13.sub.1, 13.sub.2, . . . are set on the substrate holders 12.sub.1, 12.sub.2, . . . received in the substrate container 11. On the other hand, a predetermined quantity of GaP and Ga are received in the Ga solution container 14. After internal spaces of the respective containers 11, 14 are replaced by an inert gas (Ar, for example), the containers 11, 14 are tightly closed or sealed and then set at a predetermined position within a reacting tube of an electric furnace. In this instance, the substrate container 11 is disposed above the Ga solution container 14.
The aforesaid closed condition is maintained until the boat is disassembled after completion of the liquid phase growth process, so that throughout the liquid phase growth process, the gas flowing within the reacting tube is prevented from flowing into and out from the closed boat (namely, the substrate container 11 and the Ga solution container 14).
Then, while the Ar gas is continuously flowing, the temperature is elevated to 1050.degree. C. and subsequently maintained for more than 60 minutes to ensure that GaP is sufficiently dissolved in Ga, thereby a saturated Ga solution 15 at 1050.degree. C. being produced. In this instance, the grooves 17.sub.1, 17.sub.2 in the sliding plate 16 are disposed within the containers 11, 14.
Subsequently, the boat is half-revolved in the arrowed direction a, as shown in FIG. 7, so that the Ga solution 15 flows from the Ga solution container 14 through the grooves 17.sub.1, 17.sub.2 into the substrate container 11 whereupon the Ga solution and the substrate crystals 13.sub.1, 13.sub.2, . . . are brought into contact with each other. After they are closely contacted together, the sliding plate 16 is slid to move the portion without the grooves 17.sub.1, 17.sub.2 into both containers 11, 14, thereby separating the substrate container 11 and the Ga solution container 14. Accordingly, the two containers 11 and 14 are each held in a closed or isolated condition, so that the Ga solution 15 once flown into the substrate container 11 cannot flow out even when the boat is revolved or oscillated during meltback or liquid phase growth (crystal growth) condition.
Thereafter, the temperature is elevated by 5.degree.-10.degree. C. for meltback. In the course of the temperature rises, the boat is half-revolved to make the meltback quantity uniform.
Following the foregoing temperature elevation, the boat is cooled down to 800.degree. C. to precipitate GaP in the Ga solution 15 onto the substrate crystals 13.sub.1, 13.sub.2, . . . , thereby growing a GaP growth layer. During that time, in order to make the GaP growth layer uniform in thickness, the boat is half-revolved again when the temperature drops to about 975.degree. C.
When the temperature falls to 800.degree. C., the sliding plate 16 is slid to move the grooves 17.sub.1, 17.sub.2 into the containers 11, 14, and then the boat is half-revolved for causing the Ga solution 15 to move from the substrate container 14 back to the Ga solution container 14 (to separate the Ga solution 15 from the substrate crystals 13.sub.1, 13.sub.2, . . . ), thus completing the liquid phase growth process of the GaP layer.
In the liquid phase epitaxial growth process, an oxide film, if formed on the surface of each crystalline substrate or the surface of a solution for liquid phase growth, may cause negative crystal defects in a growth layer. Accordingly, in order to obtain a growth layer with little defects, a heat-treatment in a gaseous atmosphere containing H.sub.2 must be performed to remove the oxide film from the solution surface and the substrate surface before the solution is brought into contact with the substrate.
However, the conventional closed rotary boat is so constructed as to prevent gas from flowing into and out from the deposition chamber (substrate container) and the solution chamber (solution container). With the liquid phase growth boat thus constructed, the substrates and the solution are brought into contact with each other while oxide films are still present on the substrates and the solution surface, and after the lapse of a predetermined time, meltback and/or liquid phase growth (crystal growth) is performed with the result that a number of surface defects is formed on and/or in a growth layer.